This invention generally relates to protective layers for ink jet print heads, and, more particularly, to the provision of a non-wetting protective layer for preventing the drying and accumulation of ink around the nozzles of such print heads which would otherwise interfere with the printing operation.
Ink jet printing is a non-impact technique for producing images by the deposition of ink droplets on a substrate (which may be paper, transparent film, fabric, etc.) in response to digital signals. Ink jet printers have found broad applications across markets ranging from industrial labeling to short-run printing to desktop documents and pictorial imaging.
Conventional continuous ink jet printing utilizes electrostatic charging tunnels that are placed close to the point where the ink drops are formed in a stream. The xe2x80x9ctunnelsxe2x80x9d impart an electrical charge to some of the drops so that the resulting stream consists of a mixture of charged and uncharged drops. The charged drops may be deflected downstream by the presence of deflector plates that have a large potential difference between them. A gutter, sometimes known as a xe2x80x9ccatcher,xe2x80x9d may be used to intercept the charged drops while the uncharged, undeflected drops are free to strike the recording medium. If there is no electric field present, or if the drop break-off point is sufficiently far from the electrical field (even if a portion of the stream before the drop break-off point is in the presence of an electrical field) then charging will not occur and all of the ink drops will strike the recording medium. In this manner, modulation of the intensity and distance of the electric field with respect to the stream of ink droplets modulates the density of ink deposition on the medium.
Inks for high-speed ink jet drop printers must have a number of special characteristics. Such inks must be electrically conductive, having a resistivity below about 5,000 ohm-cm, and preferably below about 500 ohm-cm. For good fluidity through small nozzles, such inks must have a viscosity in the range between 1 and 15 centi-poses at 25xc2x0 C. Typically, water-based inks are used because their inherent conductivity and viscosity is within the ranges required for operability. In addition to conductivity and fluidity, the inks must be stable over long periods of time, compatible with ink jet materials, free of microorganisms, smear resistant after printing, fast-drying on paper, and waterproof after drying.
In recent years, in order to produce higher resolution and higher quality prints, the nozzle openings in the print heads of ink jet printers have become smaller so that the printers can generate smaller ink drop sizes. Unfortunately, these smaller nozzle openings are more sensitive to the accumulation of deposits from dried out water based inks and other contaminations. Such deposits can adversely affect both the size and placement accuracy of the ink jet drop, and even plug the nozzle opening completely. This sensitivity has spawned the development of a number of devices and techniques in the prior art for preventing such deposits and consequent nozzle plugging from occurring.
One approach to the nozzle plugging problem has been the provision of devices for applying anti-wetting solvents to the print head between print runs to prevent ink from accumulating around the nozzle openings. For example, U.K. patent application GB2203994 to Takahashi et al. discloses an applicator for applying anti-wetting compositions to the nozzles on the face of a print head of an ink drop printer. The print head, which is reciprocably movable across the face of a platen, is periodically moved to one end of the platen where the applicator is placed. The applicator includes an extendable pad which then wipes the face of the print head. Similarly, European patent application 0621136 to Claslin et al. discloses a wet wipe maintenance device for a full width ink jet printer. A shuttle is mounted on a track to move along a fixed path parallel to an array of nozzle openings present in the surface of a print head. Mounted on the shuttle are an applicator for applying a liquid to the nozzle openings and a vacuum device for applying suction to the openings. The applicator is a wick of urethane felt through which water is supplied. U.S. Pat. No. 4,306,245 to Kasugayama et al. also discloses a device for cleaning discharge nozzles of an ink jet print head. When the print head moves to a print scanning region, ink in the nozzles is discharged into an opening leading to an ink recovery tank to clear them. Ink adhering around the discharge nozzles is then rubbed off by a liquid absorber fitted into the device.
Another approach to eliminating or at least ameliorating the nozzle plugging problem has been the development of new ink compositions which are less apt to build up deposits around the nozzles in the print head. For example, Carlson et al. U.S. Pat. No. 5,725,647 discloses a pigmented ink formed from an aqueous medium having dispersants for reducing the agglomeration of pigment particles in order to reduce or eliminate the deposition of foreign substances on heater elements during the jetting process. Similarly, Yamashita et al. U.S. Pat. No. 5,431,722 discloses an ink for ink jet printing comprising water, a colorant and a water soluble organic solvent and an amine for reducing clogging and unevenness of jetting.
Finally, U.S. Pat. No. 5,350,616 to Pan et al. discloses a composite orifice plate for an ink jet printer having a non-wettable layer of polymer material over the outside surface of the print heat for eliminating xe2x80x9cink puddlingxe2x80x9d which can occur on the plate and create a misdirection of spraying ink droplets during ejection.
Unfortunately, all of the aforementioned solutions to nozzle clogging have their shortcomings. For example, mechanical wiping devices add to the complexity and the expense of manufacturing the nozzle jet printer, and are not completely reliable in eliminating the ink deposits which cause clogging. Similarly, while some of the clogging problems may be ameliorated by the use of anti-clogging ink compositions, such inks have failed to eliminate the problem entirely. While the use of non-wettable polymeric materials offers some relief from the clogging problem, it has created other problems. For example, when the entire nozzle plate is formed from such a polymer, the interior of the surface of the resulting nozzles is not adequately wettable, which makes it difficult to modulate ink droplets of uniform size therethrough. The durability of the resulting nozzle plate is also reduced since such polymeric materials are softer and less wear resistant than metallic materials. Finally, few polymers will withstand the high temperatures needed for the fabrication of piezo actuators.
Clearly, what is needed is an improved nozzle plate which is not dependent upon the use of a mechanical wiping device to prevent potentially clogging deposits of dried ink from forming in the vicinity of the ink jet nozzles. Ideally, the outer and inner surface of such a nozzle plate could be formed from a metal or metal alloy to maintain the durability of the print head, and wettability of the nozzle interiors. It would further be desirable if the print head could be easily manufactured using readily accessible and inexpensive materials.
Generally speaking, the invention is an ink jet print head that eliminates or at least ameliorates all of the aforementioned clogging problems associated with prior art print head plates. Structurally, the invention is an ink jet print head that comprises a nozzle plate having an outer metal layer that includes nozzles for ejecting ink drops and a coating of a non-wetting polymer that is chemically bound to the outer surface of the metal layer of the plate. The non-wetting polymer includes at least one type of chemical group that ionically or datively bonds with the metal forming the nozzle plate. In the preferred embodiment, the non-wetting polymer is a block polymer having a head that includes the aforementioned chemically bonding chemical group, and a tail that is hydrophobic. The polymers forming the coating inherently arrange themselves into a dense array throughout the entire outside surface of the metal layer of the print head so as to provide a strongly bonded, non-wetting layer around the vicinity of the plate nozzles that resist the accumulation and drying of ink in these areas.
The metal forming the nozzle plate may be an alloy of gold, silver, or cadmium, and the coating polymer may include a chemical group that contains sulfer, selenium, or tellurium. The metal forming the nozzle plate may also be an alloy of one of the group consisting of aluminum, silicon, indium, scandium, hafnium, titanium, and zirconium, and the coating polymer may include siloxane groups. The metal layer may also be formed from an alloy including platinum, palladium, nickel, cobalt, or iridium, and the polymer may have pendant or chain carbon-carbon double bond for chemically bonding to the surface of the nozzle plate.
While the invention is described with reference to a piezoelectric ink jet print head, it is compatible with thermal or any other types of ink jet print heads, including but not limited to drop-on-demand ink jet printers.
The non-wettability of the exterior polymeric coating virtually eliminates the opportunity for liquid ink to cling to the nozzle plate, dry, and form ink jet clogging deposits. While it would be, of course, possible to fabricate the entire nozzle plate from a non-wettable polymer, such plates do not inherently provide a wettable inner surface for the ink ejecting nozzles, which in turn interferes with the reliability and control of the printing operation. The invention, by maintaining the use of a layer of metal in the nozzle plate, inherently provides for a wettable surface for the inner surfaces of the nozzle. The use of a metal layer in lieu of a polymer layer provides for a harder and more durable nozzle plate. Finally, the chemical bonding between the polymeric coating and the outer surface of the metallic nozzle plate makes it difficult to abrade the coating away from the surface of the metal in the event that auxiliary wiping devices are used in conjunction with the print head.